As shown in FIG. 1, the existing high voltage start-up circuit includes a transistor M1, a resistor R1, a diode D1 and a VDD detection unit. The connection relationship between the elements is described as follows: the drain of the transistor M1 is connected with a first end of the resistor R1 and with the pin HV; the gate of the transistor M1 is connected with a second end of the resistor R1, the cathode of the diode D1, and the drain of the transistor M3; the source of the transistor M1 is connected with the input end of the VDD detection unit 101, the pin VDD, and the capacitor C1 connected with the pin VDD; the anode of the diode D1 is connected with the source of the transistor M3 and the power supply ground. The gate of the transistor M3 is connected with the output end of the VDD detection unit 101; the output end of the PWM control module 2 is connected with the input end of the driving module 3. The output end of the driving module 3 is connected with the gate of the power transistor M2; the drain of the power transistor M2 is connected with the pin SW; the source of the power transistor M2 is connected with the first end of the resistor Rs, and the second end of the resistor Rs is connected with the power supply ground. The working principle is as follows: when the power system is starting up, the transistor M1 is conductive, and a relatively large flow of current IHV from the pin HV of the power supply chip charges the external capacitor C1 connected with the pin VDD quickly, so as to start the power system rapidly; when the potential of the pin VDD is charged to a certain voltage threshold, the transistor M3 is conductive, and the potential of the gate of the transistor M1 is pulled low, and the transistor M1 is off; the starting process ends; the PWM control module 2 and the driving module 3 enter normal working states, the power transistor M2 starts normal operation, and the power system starts to output energy.
FIG. 2 shows the working waves of the high voltage start-up circuit above. At the instant t1, the power system is powered on, and the power system enters a start-up state. The output signal VDD_OK of the VDD detection unit 101 remains a low level, and the transistor M3 is off. The current IHV charges the external capacitor C1 connected with the pin VDD of the power supply chip, and the charging current value is I1. The voltage of the pin VDD increases gradually. In the starting process, the GATE signal output by the driving module 3 remains a low level, and the power transistor M2 remains off. At the instant t2, the voltage of the pin VDD rises to a certain value, and VDD_OK is inversed to be a high level; the transistor M3 is conductive; the potential of the gate of the transistor M1 is pulled low and the transistor M1 is off, and the current IHV is reduced to zero. At the same time, the PWM control module 2 and the driving module 3 enter normal working states, the power transistor M2 starts normal operation, and the current IVDD rises from zero to a higher value of I2. At the instant t3, the power enters a standby mode, and the current IVDD reduces to a lower value I3, wherein, I2>I3>0. In the process of startup-normal operation-standby, current flowing through the resistor R1 always exists, and the current value is very small, which is close to the zero current.
In conclusion, when the power supply is in normal operation, only a smaller flow of current through the resistor R1 and only the power consumption generated by the current exist in the high voltage start-up circuit, thereby reducing the input power consumption of the power supply in normal working state. Nevertheless, as the high voltage start-up circuit and the power supply are in working states when the power system is in standby mode, the power system utilizing this high voltage start-up circuit has the problems that the standby input power consumption is rather large (over 50 mW) or the standby input current is rather large (over 300 μA), which causes energy waste, and is not suitable for occasions which require extremely low standby input power consumption or extremely low standby input current.